Impurity controlled Superconductivity/Spin Density Wave interplay in the organic superconductor : (TMTSF)_2ClO_4

– The study of the anion ordered (TMTSF )2ClO4(1−x)ReO4x solid solution in the limit of a low ReO− 4 substitution level (0 ≤ x ≤ 17%) has revealed a new and interesting phase diagram. Superconductivity is drastically suppressed as the effect of ReO− 4 non magnetic point defects increases following the digamma behaviour for usual superconductors in the presence of paramagnetic impurities. Then, no long range order can be stabilized above 0.1K in a narrow window of substitution. Finally, an insulating SDW ground state in ReO− 4 -rich samples is rapidly stabilized with the decrease of the potential strength leading to the doubling of the transverse periodicity. This extensive study has shown that the superconducting order parameter must change its sign over the Fermi surface. The symmetry of the superconductivity order parameter remains a controversial question in organic superconductors where unlike the high Tc cuprates phase sensitive experiments have failed so far to provide a clear cut answer [1]. Among the various experimental approaches used to tackle the problem of the order parameter symmetry the sensitivity of the superconducting state to non magnetic impurities is known to be quite pertinent since unlike conventional s-type superconductors, unconventional superconductors, singlet d [2] or triplet p or f type [3–6] are strongly affected by their presence. Organic superconductors belonging to the (TMTSF )2X family of one-dimensional conductors [7] provide a situation which is very well adapted to the study of the role of non magnetic impurities on the superconductivity instability. The solid solution (TMTSF )2ClO4(1−x)ReO4x which has been the subject of an exhaustive crystallographic study [8] represents a very good example since a small amount of ReO− 4 anions does not disturb the electronic structure and yet profoundly affects the stability of the superconductivity phase of the pure (TMTSF )2ClO4 compound [9,10]. A preliminary study of (TMTSF )2ClO4(1−x)ReO4x has been carried on previously [11] but it was restricted to the low dilution regime namely ≤ 6 %. It was shown, plotting the value of Tc versus the residual resistivity of the normal state that the strong suppression of Tc does not support the existence in these organic superconductors of an order parameter keeping the same sign over the entire Fermi surface. The present work extends the investigation towards higher concentrations of ReO− 4 anions. Furthermore, a reinvestigation of the procedure used for the plot of